Mmm, good to know about the other updates. So, a couple things.
First, as far as I know we currently do not specifically utilize the "peek" feature. It is a thing the operating system automatically does to every application, at least at a basic level, and then I believe application developers can choose to take greater advantage of it. Again as far as I know we do not do the latter, and so nothing should have changed in that regard.
However the responsiveness of TG itself will no doubt influence whether the application state "screenshot" that the peek feature uses is made available in a timely manner to the OS. In other words if Terragen is working super hard and its own UI thread is not getting enough CPU cycles, it could fail to provide a thumbnail in an appropriate time frame. This goes back to the general performance issue you're seeing.
I would suggest that it may be at least as likely that Windows changed application priorities and/or threading behavior for CPU-demanding, foreground applications as it is that TG changed. I'll see if I can see anything in the change log that might have affected this, but certainly between .17 and .18 there should not be.
With that in mind I'd suggest simply changing the number of CPU threads that TG uses by default. You can do this in the Preferences. Set it to 1 fewer threads than you actually have in your system (remember that threads is not the same as cores though, with hyperthreading). Let us know if that helps.
- Oshyan